U.S. Pat. No. 4,785,447 describes one type of transmultiplexer for a N-channel FDM signal wherein; for demodulating that signal, quadrature versions of a carrier are supplied from a local oscillator and a p/2 phase shifter to two mixers to produce complex signals. These complex signals are then applied through low-pass filters to analog-to-digital converters for conversions into digital form before being fed to a switching circuit. Another type of demodulating device is described in U.S. Pat. No. 4,754,449 which transforms NFDMA channels of PSK signals into a single time division multiplex (TDM) channel of PSK data. A single demodulator demodulates the single TDM channel before it is fed to a demultiplexer.
U.S. Pat. No. 4,648,100 descries a demodulation device for a single 64QAM signal in which modulated IF quadrature-related carriers are applied two demodulators. A local oscillator applies a signal, directly and through a p/2 phase shifter, to the demodulators to form two digital data channels. In a phase detector, signals from the in-phase and quadrature rails are applied to analog-to-digital converters which are also supplied with threshold adjust signals obtained from the data detector circuitry. The analog-to-digital converters in the U.S. Patent are part of a phase detector circuit which provides a phase error signal. The value and polarity of the phase error signal align the receiver-generated carriers from the local oscillator and p/2 phase shifter to the quadrature-related carriers in the input signal. This patent along with the previously mentioned patents serve to illustrate several different types of communication systems and apparatus for processing signals in these types of communication systems.